Electromagnetic (EM) waves in the millimeter-wave band (30-300 GHz) are being studied increasingly for applications to a high-speed wireless LAN (Local Area Network), radar for the prevention of vehicle collisions, or the like. Frequencies of 35, 94, and 140 GHz in particular are those of so-called “window of the air” and Electromagnetic waves at these frequencies have high transparency in the air and, therefore, the millimeter-wave band can be suitably used for the wireless LAN, radar, or the like. Moreover, when the millimeter-wave band is put to practical use, even in the case of such millimeter-wave band, EMC (Electromagnetic compatibility) problems cannot be avoided and there is no doubt that, as a countermeasure against the EMC problem, a device such as a wave absorber and nonreciprocal device is required. However, a magnetic material capable of effectively suppressing the EMC problem in the millimeter-wave band has been not yet reported to date. Conventionally, as a nonreciprocal device for countermeasures against the EMC problem, a garnet-type ferrite or spinel-type ferrite nonreciprocal device has been widely used. However, if such magnetic materials as above are used in the millimeter-wave band, a very large permanent magnet for magnetization is indispensable, which causes a magnetic circuit to be made large in size and, as a result, a problem related to practical usability has arisen in terms of the achievement of the miniaturization of the nonreciprocal device to be used.
In such a circumstance, the development of excellent magnetic materials usable for the countermeasure against the EMC problem is attracting attention. Particularly, the advent of a magnetic material having high coercivity and exhibiting high resonance frequency is expected. To meet such a demand, ε-Fe2O3 (ε-phase hematite) in the form of a single-phase nanoparticle being 100 nm in size has been produced in recent years. The nanoparticle has a characteristic of having high coercivity at room temperature.
Two kinds of substances with the chemical formula of Fe2O3 are known, one being γ-phase hematite and another being α-phase hematite. As an intermediate substance between the above two substances, ε-phase hematite had been earlier reported, however, it had been reported that the substance was intermediate phase hematite and there was no report until lately that the substance was obtained as a single phase hematite. The reason is that the ε-phase hematite is in a metastable phase and exists only under special conditions. Recently, one of the inventors of the present invention found that the single nanoparticle could be produced in a stable state by a method obtained by combining a reverse micelle method and a sol-gel method and disclosed the finding in Non-Patent References 1 to 4    Non-Patent Reference 1: Kuroki, Sakurai, Hashimoto, and Ohkoshi; “Control of spin reorientation phenomenon in ε-Fe2O3 nanomagnet” Digest of the 29th Annual Conference on Magnetics in Japan [2005], 21 pPS-16.    Non-Patent Reference 2: Sakurai, Oda, Nuida, Hashimoto, and Ohkosi; “Large coercive field and spin reorientation phenomenon in ε-Fe2O3 nanorod” Digest of the 29th Annual Conference on Magnetics in Japan [2005], 21 pPS-17.    Non-Patent Reference 3: Sakurai, Shimoyama, Hashimoto and Ohkoshi; “Preparation of magnetically oriented ε-Fe2O3 nanoparticles exhibiting large coercive field” Digest of the 30th Annual Conference on Magnetics in Japan [2006], 13 pD-3.    Non-Patent Reference 4: Ohkoshi; “Oxide nanoparticle”, Ceramics 41 [2006] No. 4, pp. 296-299.